/Engine/Source/Runtime/Engine/Classes/Components/SplineComponent.h
Given a time from 0 to the spline duration, return the spline's tangent there.
FVector GetTangentAtTime ( float Time, [ESplineCoordinateSpace::Type](API\Runtime\Engine\Components\ESplineCoordinateSpace__Type) CoordinateSpace, bool bUseConstantVelocity ) const